[clang] [clang-tools-extra] [clangd] Retrieve documentation for member function instance from index (PR #153337)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> CodeCompleteTests.cpp is failing on one of the buildbots (https://lab.llvm.org/buildbot/#/builders/25/builds/12099/steps/11/logs/stdio), which I suspect might be related to this pull request. Could you please take a look?

I had a look. While the failing test is in the same file as one modified by this PR, it's (1) in a different test that I don't expect to be affected by this PR, and (2) it's a timeout whose failure is more likely to indicate an unrelated intermittent issue.

A [subsequent build on the same buildbot](https://lab.llvm.org/buildbot/#/builders/25/builds/12101) is successful. I'll keep an eye on a few more builds, but I suspect the linked failure is a pre-existing intermittent and the fact that it's in the same file that's touched by this PR was coincidental.

https://github.com/llvm/llvm-project/pull/153337